A useful technique for permanently attaching wire conductors to conductive pads involves so-called ultrasonic bonding. A machine to perform this operation includes a transducer-mounted tool having a grooved tip by which the wire is pressed against the pad to which it is to be bonded. The tip and wire are vibrated by the transducer either transversely or parallel to the wire's axis at high frequency with a very short excursion. It is believed by some that heat generated by the resulting vibrational friction causes the metallic conductor to fuse to the pad. Others disagree as to the accuracy of this theory. At any rate, when properly executed ultrasonic bonding forms a strong, low-resistance connection between the wire and the pad relatively cheaply and quickly, compared to soldering or other bonding techniques. The process is discussed in various places, U.S. Pat. No. 3,690,538 (Gaiser et al.) being one.
The two important physical characteristics of a good wire to pad bond, ultrasonically created or otherwise, are contact resistance and pull strength. High pull strength is valuable since it allows the joint to better survive rough handling during both assembly and use. In ultrasonic bonding, pull strength is limited both by deformation of the wire during the bonding operation which weakens the wire and by the inherent strength of the bond itself. Therefore, one measure of the effectiveness of a particular bond is its actual pull strength. Contact resistance is usually very repeatable once a manufacturing operation has stabilized.